The system Na2CO3–CaCO3 at 3 GPa
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It was suggested that alkali–alkaline earth carbonates may have a substantial role in petrological processes relevant to metasomatism and melting of the Earth’s mantle. Because natrite, Na2CO3, Na–Ca carbonate (shortite and/or nyerereite), and calcite, CaCO3, have been recently reported from xenoliths of shallow mantle (110–115 km) origin, we performed experiments on phase relations in the system Na2CO3–CaCO3 at 3 GPa and 800–1300 °C. We found that the system has one intermediate compound, Na2Ca3(CO3)4, at 800 °C, and two intermediate compounds, Na2Ca(CO3)2 and Na2Ca3(CO3)4, at 850 °C. CaCO3 crystals recovered from experiments at 950 and 1000 °C are aragonite and calcite, respectively. Maximum solid solution of CaCO3 in Na2CO3 is 20 mol% at 850 °C. The Na-carbonate–Na2Ca(CO3)2 eutectic locates near 860 °C and 56 mol% Na2CO3. Na2Ca(CO3)2 melts incongruently near 880 °C to produce Na2Ca3(CO3)4 and a liquid containing about 51 mol% Na2CO3. Na2Ca3(CO3)4 disappears above 1000 °C via incongruent melting to calcite and a liquid containing about 43 mol% Na2CO3. At 1050 °C, the liquid, coexisting with Na-carbonate, contains 87 mol% Na2CO3. Na-carbonate remains solid up to 1150 °C and melts at 1200 °C. The Na2CO3 content in the liquid coexisting with calcite decreases to 15 mol% as temperature increases to 1300 °C. Considering the present and previous data, a range of the intermediate compounds on the liquidus of the Na2CO3–CaCO3 join changes as pressure increases in the following sequence: Na2Ca(CO3)2 (0.1 GPa) → Na2Ca(CO3)2, Na2Ca3(CO3)4 (3 GPa) → Na4Ca(CO3)3, Na2Ca3(CO3)4 (6 GPa). Thus, the Na2Ca(CO3)2 nyerereite stability field extends to the shallow mantle pressures. Consequently, findings of nyerereite among daughter phases in the melt inclusions in olivine from the sheared garnet peridotites are consistent with their mantle origin.
KeywordsNa–Ca carbonates High-pressure experiment Nyerereite Shortite Raman Carbonatite
The authors are very grateful to Robert Luth and an anonymous reviewer for constructive comments and helpful suggestions and Taku Tsuchiya for editorial handling. We thank Kathryn Moore and an anonymous reviewer for critical reading of an early draft of the manuscript. This study was financially supported by the Russian Foundation for Basic Research (Project No 17-05-00501). KL thanks for partial support from the state assignment Project No. 0330-2016-0006.
- Li Z (2015) Melting and structural transformations of carbonates and hydrous phases in Earth’s mantle, Doctor of Philosophy (Geology). The University of Michigan, United StatesGoogle Scholar
- Menzies M, Hawkesworth C (1986) Mantle metasomatism. Academic Press, LondonGoogle Scholar
- Rashchenko SV, Bakakin VV, Shatskiy AF, Gavryushkin PN, Seryotkin YV, Litasov KD (2017) Noncentrosymmetric Na2Ca4(CO3)5 carbonate of “M13M23XY3Z” structural type and affinity between borate and carbonate structures for design of new optical materials. Cryst Growth Des 17(11):6079–6084CrossRefGoogle Scholar
- Sharygin I, Litasov K, Shatskiy A, Golovin A, Ohtani E, Pokhilenko N (2015) Melting phase relations of the Udachnaya-East group-I kimberlite at 3.0–6.5 GPa: experimental evidence for alkali-carbonatite composition of primary kimberlite melts and implications for mantle plumes. Gondwana Res 28:1391–1414CrossRefGoogle Scholar
- Shatskiy A, Gavryushkin PN, Sharygin IS, Litasov KD, Kupriyanov IN, Higo Y, Borzdov YM, Funakoshi K, Palyanov YN, Ohtani E (2013a) Melting and subsolidus phase relations in the system Na2CO3-MgCO3+-H2O at 6 GPa and the stability of Na2Mg(CO3)2 in the upper mantle. Am Miner 98(11–12):2172–2182CrossRefGoogle Scholar
- Soltys A, Giuliani A, Phillips D, Kamenetsky VS, Maas R, Woodhead J, Rodemann T (2016) In-situ assimilation of mantle minerals by kimberlitic magmas—direct evidence from a garnet wehrlite xenolith entrained in the Bultfontein kimberlite (Kimberley, South Africa. Lithos 256:182–196CrossRefGoogle Scholar